The present invention relates in general to a diode providing electrostatic discharge (ESD) protection, and in particular to a diode manufactured by bipolar complementary metal oxide semiconductor (BiCMOS) processes.
Among ESD protection devices, a diode has one of the simplest structures. Properly forward biasing a diode during an ESD event requires only a small silicon area for effective protection. FIG. 1 is a cross section of a conventional diode formed with a heavy-doped P-type (P+) region and an N-type well (NW).
FIG. 2 shows a conventional diode string, connected between high voltage (Vhigh) and low voltage (Vlow) power lines, serving as an ESD protection device. The conventional diode string consists of several diodes connected in series. As shown in FIG. 2, a parasitic Darlington amplifier is formed in the diode string by the series-connected parasitic PNP bipolar transistors (BJTs), resulting in substrate current leakage forward to the grounded P-type (P) substrate. This substrate current leakage becomes more severe when operating temperature or diode number in the diode string increases.
There are several solutions for substrate current leakage. FIG. 3 shows a conventional circuit schematic suppressing substrate current leakage, in which the bias circuit 8 conducts a small amount of forward current to a lower portion of a diode string. Experimental results from the circuit indicate that, at high temperature, substrate current leakage still occurs at significant magnitude.
Another solution involves modification of the structure of each diode. By lowering the common-base gain of each parasitic BJT, the overall current gain of a Darlington amplifier is decreased, reducing substrate current leakage. Alternatively, eliminating formation of a Darlington amplifier, the substrate current leakage of a diode string may be substantially diminished. FIG. 4 shows a diode string with diodes of an exemplary diode structure. In comparison with the diode of FIG. 1, each diode in FIG. 4 has not only a P+ region and an NW, but also a deep NW under the NW. The common-base current gain of each parasitic BJT is reduced by the enlarged base width thereof. The overall current gain of a Darlington amplifier is accordingly decreased, and the substrate current leakage is reduced.
FIG. 5 shows another diode string in which, for each diode, the P-type well (PW) is completely enclosed by an NW and a deep NW. Through a wire connection, the voltage potentials of the NW and the deep NW are the same as that of the anode of the enclosed diode. In FIG. 5, because the base and emitter of each parasitic PNP BJT are at the same voltage potential and the emitter, the PW, has much lower doped concentration, the common-base current gain of each parasitic PNP BJT is decreased, such that each parasitic PNP BJT is difficult to activate. Furthermore, the Darlington amplifier is not formed, due to the modification of the diode structure, improving substrate current leakage.
The diodes in FIGS. 4 and 5 are suitable for CMOS semiconductor processes providing formation of a deep NW. However, with regard to a diode string with ESD protection for SiGe BiCMOS processes, the diode string in FIG. 6 is generally utilized. In FIG. 6, each diode has a heavy-doped N-type (N+) buried layer 94. Because the emitter, the PW 90, has a much lower doping concentration, the common-base current gain of each parasitic PNP BJT is decreased. Furthermore, the Darlington amplifier is not formed, due to modification of the diode structure. Both results improve substrate current leakage.